The invention relates to a light transmitting device, and more particularly, to a light transmitting device which may be used in an illumination optical system of an endoscope or the like.
An endoscope includes an optical system which is constructed in a manner as schematically illustrated in FIG. 1. Specifically, the endoscope has a light source assembly 10 including a light source 1 such as lamp, a reflector 1a and a condenser lens 2. Light from the source 1 is condensed by the condenser lens 2 so as to impinge upon an incident end face 3a of a light guide 3 which is formed by a bundle of optical fibres. After passage through the light guide 3, the light emerges from an exit end face 3b to be directed through an illumination lens 4 to irradiate an object 5 being observed. An image of the object 5 can be observed through a path including an observation window 6a, an objective lens assembly 6, an image guide 7, formed by a bundle of optical fibers, and an eyepiece 8.
In the use of light transmitting device as illustrated by the light guide 3 mentioned above in which a number of optical fibers are bundled and bonded together by means of an adhesive, the relatively poor heat resistance of the adhesive may cause the adhesive to be scorched in the region of the incident end face 3a where intensive light from the source 1 impinges in a concentrated manner, causing a "burn-out" of the light guide. Such "burn-out" causes a reduction in the amount of light available for the illumination and causes non-uniformity of the illumination, and thus is unfavorable.
The distribution of the intensity of light which impinges upon the end face 3a of the light guide 3 used in such illumination system is illustrated graphically in FIG. 2. It will be evident from this Figure that the severity of such "burn-out" increases toward the center of the incident end face. This problem is accentuated by the fact that the diameter of the endoscopes has been gradually reduced, requiring a thinner light guide. This in turn makes it necessary to use a light source of higher intensity to maintain an equal amount of light thereby increasing the occurrence of "burn-out".
Another difficulty with the use of such an illumination system employing a light guide such as that shown at 3 relates to the alignment of individual fibres between the incident and the exit end faces 3a, 3b. Thus, if the light which impinges upon the incident end face 3a has a non-uniform distribution of intensity, such distribution is directly transmitted to the exit end face 3b, with result that the light which illuminates the object 5 being observed has a non-uniform distribution. It is also to be noted that a ray of light which emerges in the same direction as the direction of a particular fibre has a greater intensity than any other ray which emerges from the particular fibre. Accordingly, if the incident end and the exiting end of a particular fibre are displaced from each other, the refraction of rays by the illumination lens 4 causes those rays having greater or greatest intensities to be directed to one side of the object 5, as illustrated in FIG. 3, thus also causing non-uniform illumination.
In an attempt to prevent the occurrence of "burn-out" mentioned above, the use of a single fibre, formed by a material which absorbs infrared radiation and disposed between the incident end of the light guide and the light source, or the use of a ventilation cooling technique has been proposed, but such technique fails to eliminate the non-uniform illumination mentioned above.
To eliminate described disadvantages which are experienced with a light guide used as a light transmitting device, it is desirable to cause the light from a light source to impinge upon the incident end face of the light guide in a uniform manner, thus providing a uniform distribution of light intensity across such end face.